samples: sysbuild: with mcu_boot on the stm32h7rs targets

Run the sample to execute in place on the external flash of the
stm32h7s78_dk or nucleo_h7s3l8 boards.
with_boot is build and linked for the external flash
Download the mcuboot zephyr.bin in internal memory
Download the with_mcuboot zephyr.signed.bin in external memory

Signed-off-by: Francois Ramu <francois.ramu@st.com>

Author: FRASTM

samples/sysbuild/with_mcuboot/boards/nucleo_h7s3l8.overlay

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2025 STMicroelectronics
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * Define the device, controller and partition to be the external memory
+ * for running the application in external NOR from MCUboot
+ */
+/ {
+	chosen {
+		zephyr,flash = &mx25uw25645;
+		zephyr,flash-controller = &mx25uw25645;
+		zephyr,code-partition = &slot0_partition;
+	};
+};

samples/sysbuild/with_mcuboot/boards/stm32h7s78_dk.overlay

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2025 STMicroelectronics
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * Define the device, controller and partition to be the external memory
+ * for running the application in external NOR from MCUboot
+ */
+/ {
+	chosen {
+		zephyr,flash = &mx66uw1g45;
+		zephyr,flash-controller = &mx66uw1g45;
+		zephyr,code-partition = &slot0_partition;
+	};
+};